Seeing the bigger picture

You are driving on a highway, trying to recall the items you need to grab at the grocery store. Milk, eggs, tomatoes … what was the last one? Suddenly, as you focus on the list, you realize you've missed the giant exit sign.

We tend to assume that we will see things right in front of us. "But if there is some big object casting light into our eyes, and we wouldn't see it because our attention is elsewhere—that's really fascinating!" says Chaz Firestone, who directs the Johns Hopkins Perception & Mind Laboratory.

Earlier this year, Firestone and colleagues at Johns Hopkins conducted one of the largest studies of a phenomenon known as inattentional blindness, where a person fails to notice something obvious because another object or task consumes their attention. Many psychologists considered this phenomenon proof that we can be aware of something only if we pay attention to it. But the result of the new study, published May 19 in the journal eLife, challenges that assumption. Inattentional blindness became popular in 1999 when two Harvard professors published their "invisible gorilla" experiment, where participants were asked to count the number of times actors passed a basketball on a screen. About halfway through, a person in a gorilla suit marched past the actors, pounded its chest, and left the scene. When the video ended, the researchers asked participants how many passes they counted. And then they asked: "Did you notice anything unusual? Did you see a gorilla walk across the screen?" More than half the participants did not notice the gorilla.

"When your attention is soaked up by such a task, people are prone to miss really unexpected, surprising events or objects," says Ian Phillips, the senior author of the study. But Phillips, a philosopher who studies perception, suspected that when a person says, "No, I didn't see anything unusual," they might have had some experience of the object, even if it is not a full-blown image of it.

To test this hypothesis, the researchers designed a five-part experiment involving more than 25,000 online participants.

They asked people to look at an asymmetrical cross on a screen for a few seconds and judge whether the vertical or the horizontal arm is longer. After a few trials, a red line appeared on either the left or right side of the screen along with the cross. The participants were asked if they noticed anything unusual. Regardless of their responses, they were then told that there was a red line (the unexpected object) and asked to guess whether it was on the right- or left-hand side of the screen. Almost 30% of the participants said they did not notice anything unusual. But here's the twist—about 70% of these non-noticers guessed the location of the red line correctly.

"These subjects were showing that they weren't just guessing randomly— they had, in fact, picked up on information about the unexpected object," Phillips says.

The scientists repeated the experiment four more times with slight adjustments, such as asking people to guess the color of the line instead of its location or using a different object on the screen. The result was consistent—in each case, about a third of the participants said they did not notice the unexpected object. Researchers also asked participants how confident they were with their responses. Surprisingly, even subjects who were most adamant in saying they did not notice anything unusual had a better than 50/50 chance of answering correctly where the line was, Phillips says. The scientists also found a few "false alarms," where participants said they noticed something unusual, even in the absence of an unexpected object.

The findings showed that the threshold for claiming to see something could differ by individual, says Howard Egeth, a psychophysicist and co-author of the study. When a person is asked a binary question, their tendencies to say yes or no may be influenced by what they think of the responses' trade-off. Some people might say yes more often to increase their chances of getting the correct answers. The scientists showed that this is the case through a mathematical framework called signal detection theory, where they compared how conservative people answered (responding "no" when they weren't sure) and how well they executed the task (responding correctly to the questions).

To Egeth, who had started investigating the phenomenon two decades ago, the new study also confirms that one may not completely miss the thing in front of them even when they are not paying attention to it.

"It does challenge the notion that awareness requires attention," Firestone says. But the question is, "What does it look like to have residual awareness of something?"

There are also practical questions to consider, Firestone says. "It's one thing to miss a gorilla in a funny game on a video, but it's another thing to miss a pedestrian when you're driving your car or to miss a tumor when you are a radiologist looking at a scan." A better understanding of this phenomenon may help us appreciate how the mind works.